This invention relates to microprocessor controlled capacitor charge-discharge voltage-to-frequency converters and analog-to-digital converters that are particularly suitable for use with resistance bridge transducers.
Within the last ten years, circuits have been proposed for analog-to-digital conversion in which the charging and discharging of a capacitor in an integrator circuit has been used to obtain values which are proportional to the input signal voltage and which are independent of long term changes in capacitor characteristics. One such circuit is disposed in the patent to Schmidhauser U.S. Pat. No. 3,693,843. This patent discloses the use of the technique of charging the feedback capacitor used in an integrator circuit at a rate proportional to an input signal voltage and discharging it as required using a reference discharge pulse to maintain the total change in the charge on the capacitor at essentially zero. When a steady-state is reached, the frequency of the pulse train is proportional to the input signal voltage and the arrangement is a voltage-to-frequency converter. The ratio of the number of discharge pulses applied to the feedback capacitor in a measurement period to the maximum number of discharge pulses that can be applied in the measurement period is proportional to the value of the input signal voltage. When used in this mode, the circuit functions as an analog-to-digital converter.
There are many uses for voltage-to-frequency and analog-to-digital conversion circuits of this type but limitations of size and circuit complexity of the previously designed circuits are a drawback to widespread application of this technique. One such need for improved circuits is in the petroleum industry where down hole pressure and temperature measurements are frequently made using resistance bridge transducers. In this application, circuits with a small number of components, small physical size, low-power consumption from a single polarity power supply, high resolution high accuracy and the capability of functioning over a wide ambient temperature range are required.